So as we know, the job of the lungs is to transfer oxygen from the outside atmosphere into the body while also removing waste products that the body has produced and needs to get rid of- carbon dioxide. The actual gas exchange occurs in a structure called the alveoli, these are small grape like structures that allow efficient gas exchange to occur. Alveoli are surrounded by a dense network of capillaries, one cell thick blood vessels, and these carry de-oxygenated blood to the lungs. The high concentration of oxygen inside the alveoli diffuses across the thin alveoli epithelium and also the thin capillary endothelium thus entering the blood. The opposite happens with CO2 as it would diffuse in the oppoiste direction, which is from the blood to the alveoli and so it diffuses across the endothelium first then across the alveoli's epithelium where it is then exhaled out.I would then go onto talk about what makes diffuses more efficient and so on.